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The 13th termof an arithmetic sequence is 61 and the 20th term of the squence is 82. Calculate the sum of the first 25 terms of the sequence
13 years ago

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Answered by Steve
T13 = a+12d = 61
T20 = a+19d = 82
so,
T20-T13 = 7d = 21
d = 3
a = 25

S25 = 25/2 (25 + 25+24*3)
= 25/2 (122) = 25*61 = 1525

13 years ago
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The 13th termof an arithmetic sequence is 61 and the 20th term of the squence is 82. Calculate the sum of the first 25 terms of the sequence

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